Flexible inverse pyroelectric film and preparation process thereof

Abstract

The invention discloses a flexible inverse pyroelectric film and a preparation process thereof, the flexible inverse pyroelectric film comprises an inorganic nanometer filler and an organic polymer, the inorganic nanometer filler is PBZ lead barium zirconate nanofiber, the organic polymer is a PVDF flexible substrate, the chemical formula of the PBZ lead barium zirconate nanofiber is Pb < 1-x > Ba < x > ZrO < 3 >, and X is 0.2. The composite material has the advantages that the breakdown field strength can be increased, meanwhile, the dielectric constant of the composite material is improved, the composite material can be conveniently processed into various shapes due to the soft characteristic of the composite material, and integrated inverse pyroelectric material refrigeration can be further achieved easily.

Description

Technical Field

[0001] The present invention relates to the field of composite film materials, and particularly to a flexible pyroelectric thin film and a preparation process thereof. Background Art

[0002] With the improvement of material life, various electronic products have gradually entered each family, and their development direction is towards high integration, miniaturization and light weight. The high integration of chips and components generates a large amount of heat during their use. When the temperature is high, the entire device will fail or even be damaged. Therefore, when microelectronic devices are highly integrated, their heat dissipation problems must also be considered. At the same time, there are also many refrigeration requirements in daily life, industrial production, medicine, etc. At present, traditional vapor compression refrigeration is mostly used. The problem brought by this refrigeration method is at the cost of destroying the ozone layer. Although researchers are developing materials that can replace Freon, the problem of global warming still persists. Therefore, seeking a cleaner, more efficient and environmentally friendly refrigeration method is a major problem that must be considered and solved today.

[0003] Currently, new refrigeration methods are under research and development, including solar adsorption, thermoelectric, magnetic refrigeration and pyroelectric refrigeration, etc. In contrast, pyroelectric refrigeration and magnetic refrigeration have higher energy conversion efficiency, and a crucial point is their good environmental friendliness. Therefore, magnetic refrigeration and pyroelectric effect based on solid-state refrigeration are highly expected and considered to be ways to solve the refrigeration problem. Pyroelectric refrigeration and magnetic refrigeration have similar working mechanisms, but the efficiency of magnetic refrigeration is limited by the magnitude of its own magnetic field. The larger the magnetic field, the higher the efficiency, and vice versa, which goes against the trend of integrated development. Pyroelectric refrigeration has attracted wide attention due to its large refrigeration coefficient, high energy conversion efficiency and easy integration. At the same time, the acquisition and control of electric fields are easier than those of magnetic fields. Therefore, the research on refrigeration behavior based on the pyroelectric effect has important theoretical and practical values.

[0004] At the current stage, pyroelectric materials mainly include inorganic ferroelectric antiferroelectric single crystals, ceramics (bulk and thick films), thin films, and organic ferroelectric polymers, etc., and all of them have obtained satisfactory results. However, the existing problems should not be ignored. Single crystal materials are not easy to be used away from the substrate, and the production cost is relatively high; ceramic materials have a low dielectric breakdown field strength; thin film materials have problems such as small volume and low refrigeration capacity. Summary of the Invention

[0005] The purpose of the present invention is to provide a flexible pyroelectric thin film and a preparation process thereof, which can not only increase its breakdown field strength, but also improve its dielectric constant, and the soft characteristics of the composite material are also convenient for processing into various shapes, and it is easy to further realize integrated pyroelectric material refrigeration.

[0006] The above technical object of the present invention is achieved by the following technical solutions:

[0007] A flexible pyroelectric thin film, characterized in that it comprises inorganic nano-fillers and an organic polymer, the inorganic nano-fillers are lead barium zirconate titanate nano-fibers, and the organic polymer is a PVDF polyvinylidene fluoride flexible substrate, wherein the chemical formula of the lead barium zirconate titanate nano-fibers is: Pb 1-x Ba x ZrO3, where X is 0.2.

[0008] A preparation process of a flexible pyroelectric thin film, characterized in that it comprises the following steps:

[0009] Take lead acetate trihydrate Pb(CH3COO)2·3H2O and barium acetate C4H6BaO4 and dissolve them in glacial acetic acid CH3COOH and deionized water according to the corresponding molar ratio of elements. After stirring at a certain temperature and cooling to room temperature, add a certain amount of lead acetate trihydrate Pb(CH3COO)2·3H2O for the second time to obtain a first solution;

[0010] Take the corresponding mass of zirconium propoxide solution C 12 H 28 O4Zr and dissolve it in 2-ethylene glycol methyl ether C3H8O2. After stirring at room temperature, add acetylacetone C5H8O2 and stir to obtain a second solution;

[0011] Mix the first solution and the second solution, stir, and then adjust the concentration and pH value of the final lead barium zirconate titanate solution through deionized water and acid-base solution. After standing and aging, filter through a 0.2μm filter paper, then add polyvinylpyrrolidone and stir to obtain a PBZ nano-precursor through electrospinning;

[0012] Place the PBZ nano-precursor in a sintering furnace, obtain the final product through an annealing process, and cool to room temperature to obtain PBZ nano-fibers;

[0013] Disperse the PBZ nano-fibers in N, N-dimethoxymethanamide, ultrasonically treat, and then slowly add PVDF powder, and stir magnetically to obtain a composite solution;

[0014] Cast the composite solution onto a glass slide to form a thin film sample. The thin film sample is made to have a corresponding thickness by a highly controllable doctor blade. After casting, place it in a vacuum drying oven at 80°C for 20h to evaporate the solvent DMF, obtain the composite film from the glass slide, and spray a gold electrode on the surface of the composite film to obtain the flexible pyroelectric thin film.

[0015] Preferably, the stirring temperature of the lead acetate trihydrate Pb(CH3COO)2·3H2O and barium acetate C4H6BaO4 is 50-70°C, and the stirring time is 20-40min.

[0016] Preferably, the addition amount of lead acetate trihydrate Pb(CH3COO)2·3H2O is 15-25% of the mass of the first time.

[0017] Preferably, the zirconium propoxide solution C 12 H 28 O4Zr is dissolved in 2-ethoxyethanol C3H8O2 for 5-15 min of stirring, and acetylacetone C5H8O2 is added for 20-40 min of stirring.

[0018] Preferably, the stirring time of the first solution and the second solution is 1.5-2.5 h, the concentration of the lead barium zirconate titanate (PBZ) solution is 0.2-0.8 mol / L, and the pH value is 2.2-6.2.

[0019] Preferably, the electrospinning parameters are: the spinning voltage is 11 kV, the spinning current is 1.7 A, the distance from the spinning needle to the collector is 8 cm, and the spinning rate is 0.7 mL / h.

[0020] Preferably, the annealing process is specifically as follows: first, it is heated from room temperature to 350 °C at a heating rate of 3 °C / min, and kept at 350 °C for 30 min. Then, the heating temperature is raised from 350 °C to 450 °C at a heating rate of 5 °C / min, and kept at 450 °C for 60 min. Finally, the temperature is raised from 450 °C to the final temperature at a heating rate of 5 °C / min and kept at a constant temperature for 3 h. The final annealing temperature ranges from 550 °C to 750 °C, the temperature difference is 50 °C, and the holding time is 3 h.

[0021] Preferably, the addition amount of the PBZ nanofibers is 1-6 vol% of the total volume, the ultrasonic treatment time is 0.5-1.5 h, the magnetic stirring temperature is 30 °C, and the stirring time is 20 h.

[0022] Preferably, the thickness of the thin film sample is 0.1-0.4 mm, the temperature of the vacuum drying oven is 80 °C, and the drying time is 20 h.

[0023] In summary, the present invention has the following beneficial effects:

[0024] 1. The present invention provides a method for obtaining PBZ precursor nanofibers with uniform thickness and smooth surface, forming uniform cylindrical fibers by regulating the solution concentration and pH value.

[0025] 2. The annealing temperature of the present invention can obtain a pure perovskite structure, with strong characteristic diffraction peaks, elimination of lead oxide (PbO) impurities, and formation of pure polycrystalline PBZ nanofibers.

[0026] 3. The present invention provides a method for changing the structural morphology of PBZ precursor nanofibers by regulating the pH value of the solution. When the pH is 4.2, the nanofibers are thinner, with uniform size, a diameter of about 245 nm, and a length of 700 nm.

[0027] 4. For the composite films with different volume ratios introduced in the present invention, the characteristic peaks corresponding to PBZ can be measured. With the increase of the volume doping ratio, the characteristic peaks of PBZ become higher and higher, the number of PBZ nanofibers contained per unit area gradually increases, the thickness is basically uniform, the length is basically the same, the dispersion is better, and there is no large-area agglomeration phenomenon. Moreover, the fibers are coated on the inside and surface of the film in different postures, and the surface quality of the PBZ / PVDF flexible pyroelectric film is better. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the following drawings are only some embodiments of the present invention. For those skilled in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0029] Figure 1 It is a schematic diagram of the preparation process of a PBZ / PVDF flexible pyroelectric film of the present invention;

[0030] Figure 2 It is the surface morphology of PBZ precursor nanofibers at different concentrations of the present invention;

[0031] Figure 3 It is the XRD pattern of PBZ nanofibers at different annealing temperatures of the present invention;

[0032] Figure 4 It is the crystal structure and morphology test at different pH values of the present invention;

[0033] Figure 5 It is the XRD pattern, SEM micrograph and infrared test of the composite film of PBZ nanofibers with different volume ratios of the present invention;

[0034] Figure 6 It is the performance test of the composite film with different volume ratios of PBZ nanofibers introduced in the present invention;

[0035] Figure 7 It is the dielectric constant and dielectric loss of the 3 vol% PBZ composite film of the present invention at different temperatures and different frequencies;

[0036] Figure 8 It is the heat flow change of the 3 vol% PBZ composite film of the present invention under different electric fields;

[0037] Figure 9 This is the inverse pyroelectric performance of the pure PVDF film and the 3 vol% PBZ composite film of the present invention under different electric fields. Specific Embodiments

[0038] The following further illustrates the specific embodiments of the present invention in conjunction with the accompanying drawings. This embodiment does not constitute a limitation to the present invention.

[0039] As Figure 1 shown is a schematic diagram of a flexible inverse pyroelectric film preparation process. The preparation process includes the separate preparation of two solutions, filtration after mixing the two solutions, obtaining a PBZ nanofiber precursor through an electrospinning process, annealing to obtain PBZ nanofibers, preparing a PBZ-nfs / PVDF mixed solution, a casting process, sputtering gold after drying, and finally obtaining a PBZ / PVDF flexible inverse pyroelectric film.

[0040] The following exemplarily illustrates the preparation process of PBZ-nfs used in the embodiments:

[0041] Prepare a concentration gradient of PBZ solutions, including four categories from 0.2 mol / L to 0.8 mol / L, with a concentration difference of 0.2 mol / L;

[0042] 0.2 mol / L PBZ solution:

[0043] First solution: Weigh 1.09 g of lead acetate trihydrate and 0.15 g of barium acetate, and dissolve the two in 1.6 mL of glacial acetic acid and 1.9 mL of deionized water; after stirring the mixed solution at 60 °C for 30 min, cool it to room temperature, and add 20% more lead to make up for the volatilization of lead elements during the annealing process.

[0044] Second solution: Weigh 1.42 g of zirconium propoxide solution, dissolve it in 0.6 mL of 2-ethylene glycol methyl ether solution, stir at room temperature for 10 min, and add 0.8 mL of acetylacetone, then stir at room temperature for 30 min.

[0045] Mix the two prepared solutions containing Pb, Ba, and Zr, add 5 mL of deionized water, and stir at room temperature for 2 h;

[0046] At this time, the pH value of the solution is 4.2;

[0047] Let the mixed solution stand and age for 24 h for standby;

[0048] Filter the mixed solution using a 0.2 μm filter paper;

[0049] Add 0.65 g of PVP and stir at room temperature for 2 h;

[0050] Prepare a PBZ nano-precursor with a solution concentration of 0.2 mol / L by electrospinning. The spinning voltage is 11 kV, the spinning current is 1.7 A, the distance from the spinning needle to the collector is 8 cm, and the spinning rate is 0.7 mL / h. The surface morphology of the obtained sample on the PBZ precursor nanofibers with a concentration of 0.2 mol / L is as Figure 2 shown in a.

[0051] PBZ solution with a concentration of 0.4 mol / L:

[0052] First solution: Weigh 1.46 g of lead acetate trihydrate and 0.20 g of barium acetate, and dissolve the two in 1.6 mL of glacial acetic acid and 1.9 mL of deionized water. After stirring the mixed solution at 60 °C for 30 min, cool it to room temperature and add 20% more lead to compensate for the volatilization of lead elements during the annealing process.

[0053] Second solution: Weigh 1.87 g of zirconium propoxide solution and dissolve it in 0.6 mL of 2-ethoxyethanol solution. Stir it at room temperature for 10 min, add 0.8 mL of acetylacetone, and stir it at room temperature for 30 min.

[0054] Mix the two prepared solutions containing Pb, Ba, and Zr, add 5 mL of deionized water, and stir it at room temperature for 2 h;

[0055] At this time, the pH value of the solution is 4.2;

[0056] Let the mixed solution stand and age for 24 h for standby;

[0057] Filter the mixed solution using a 0.2 μm filter paper;

[0058] Add it to 0.65 g of PVP and stir it at room temperature for 2 h;

[0059] Prepare a PBZ nano-precursor with a solution concentration of 0.4 mol / L by electrospinning. The spinning voltage is 11 kV, the spinning current is 1.7 A, the distance from the spinning needle to the collector is 8 cm, and the spinning rate is 0.7 mL / h. The surface morphology of the obtained sample on the PBZ precursor nanofibers with a concentration of 0.4 mol / L is as Figure 2 shown in b.

[0060] PBZ solution with a concentration of 0.6 mol / L:

[0061] First solution: Weigh 2.18 g of lead acetate trihydrate and 0.31 g of barium acetate, and dissolve the two in 1.6 mL of glacial acetic acid and 1.9 mL of deionized water. After stirring the mixed solution at 60 °C for 30 min, cool it to room temperature and add 20% more lead to compensate for the volatilization of lead elements during the annealing process.

[0062] Second solution: Weigh 2.82 g of zirconium propoxide solution and add it to 0.6 mL of 2-ethoxyethanol solution. Stir for 10 min at room temperature, then add 0.8 mL of acetylacetone and stir for 30 min at room temperature.

[0063] Mix the two prepared solutions containing Pb, Ba, and Zr, add 5 mL of deionized water, and stir for 2 h at room temperature.

[0064] The pH value of the solution at this time is 4.2.

[0065] Let the mixed solution stand and age for 24 h for standby.

[0066] Filter the mixed solution using a 0.2-μm filter paper.

[0067] Add it to 0.65 g of PVP and stir for 2 h at room temperature.

[0068] Prepare a PBZ nano-precursor with a solution concentration of 0.6 mol / L by electrospinning. The electrospinning voltage is 11 kV, the electrospinning current is 1.7 A, the distance from the electrospinning needle to the collector is 8 cm, and the electrospinning rate is 0.7 mL / h. The surface morphology of the obtained sample on the surface of the 0.6 mol / L PBZ precursor nanofibers is as Figure 2 shown in c.

[0069] 0.8 mol / L PBZ solution:

[0070] First solution: Weigh 2.91 g of lead acetate trihydrate and 0.41 g of barium acetate, and dissolve them in 1.6 mL of glacial acetic acid and 1.9 mL of deionized water; after stirring the mixed solution at 60 °C for 30 min, cool it to room temperature, and add 20% more lead to compensate for the volatilization of lead elements during the annealing process.

[0071] Second solution: Weigh 3.74 g of zirconium propoxide solution and add it to 0.6 mL of 2-ethoxyethanol solution. Stir for 10 min at room temperature, then add 0.8 mL of acetylacetone and stir for 30 min at room temperature.

[0072] Mix the two prepared solutions containing Pb, Ba, and Zr, add 5 mL of deionized water, and stir for 2 h at room temperature.

[0073] The pH value of the solution at this time is 4.2.

[0074] Let the mixed solution stand and age for 24 h for standby.

[0075] Filter the mixed solution using a 0.2-μm filter paper.

[0076] Add it to 0.65 g of PVP and stir for 2 h at room temperature.

[0077] Prepare a PBZ nano-precursor with a solution concentration of 0.8 mol / L by electrospinning. The electrospinning voltage is 11 kV, the electrospinning current is 1.7 A, the distance from the electrospinning needle to the collector is 8 cm, and the electrospinning rate is 0.7 mL / h. The surface morphology of the obtained sample on the PBZ precursor nanofibers with a concentration of 0.8 mol / L is as Figure 2 shown in d.

[0078] The above process is for the obtained PBZ nanofiber precursor. For illustration purposes, in this example, PBZ precursor nanofibers with a concentration of 0.8 mol / L are selected and placed in a sintering furnace. The final product is obtained through an annealing process: the annealing process first rises from room temperature to 350 °C at a heating rate of 3 °C / min and is held at 350 °C for 30 min to fully volatilize the solvent. Then the heating temperature rises from 350 °C to 450 °C at a heating rate of 5 °C / min and is held at 450 °C for 60 min to fully pyrolyze the precursor nanofibers. Finally, the temperature rises from 450 °C to the final temperature at a heating rate of 5 °C / min and is held at a constant temperature for 3 h. Here, the selected final annealing temperature ranges from 550 °C to 750 °C with a temperature difference of 50 °C and a holding time of 3 h. The XRD patterns of the PBZ nanofibers at different annealing temperatures are as Figure 3 shown.

[0079] Prepare a PBZ solution with a pH gradient. Divide it into five gradients from 2.2 to 6.2 with a difference of 1;

[0080] PBZ solution with pH = 2.2:

[0081] Select a PBZ solution with a concentration of 0.8 mol / L:

[0082] Use glacial acetic acid CH3COOH to adjust the pH value of the solution to 2.2;

[0083] Let the mixed solution stand and age for 24 h for standby;

[0084] Filter the mixed solution using a 0.2 μm filter paper;

[0085] Add it to 0.65 g of PVP and stir at room temperature for 2 h;

[0086] Prepare a nano-precursor of the PBZ solution with pH = 2.2 by electrospinning. The electrospinning voltage is 11 kV, the electrospinning current is 1.7 A, the distance from the electrospinning needle to the collector is 8 cm, and the electrospinning rate is 0.7 mL / h. The crystal structure of the obtained sample of PBZ nanofibers with pH = 2.2 is as Figure 4 shown in a, and the surface morphology of the precursor nanofibers is as Figure 4 shown in b. The morphology of the PBZ nanofibers after selecting the final annealing temperature of 750 °C is as Figure 4 shown in g.

[0087] PBZ solution with pH = 3.2:

[0088] Select a 0.8 mol / L PBZ solution:

[0089] Use glacial acetic acid CH3COOH to adjust the pH value of the solution to 3.2;

[0090] Let the mixed solution stand and age for 24 h for later use;

[0091] Filter the mixed solution with a 0.2 μm filter paper;

[0092] Add it to 0.65 g of PVP and stir at room temperature for 2 h;

[0093] Prepare a nano-precursor of PBZ solution with pH = 2.2 by electrospinning. The spinning voltage is 11 kV, the spinning current is 1.7 A, the distance from the spinning needle to the collector is 8 cm, and the spinning rate is 0.7 mL / h. The crystal structure of the obtained PBZ nanofibers at pH = 3.2 is as Figure 4 shown in a, the surface morphology of the precursor nanofibers is as Figure 4 shown in c, and the morphology of the PBZ nanofibers after selecting the final annealing temperature of 750 °C is as Figure 4 shown in h.

[0094] PBZ solution with pH = 4.2:

[0095] Select a 0.8 mol / L PBZ solution:

[0096] At this time, the pH value of the solution is 4.2;

[0097] Let the mixed solution stand and age for 24 h for later use;

[0098] Filter the mixed solution with a 0.2 μm filter paper;

[0099] Add it to 0.65 g of PVP and stir at room temperature for 2 h;

[0100] Prepare a nano-precursor of PBZ solution with pH = 2.2 by electrospinning. The spinning voltage is 11 kV, the spinning current is 1.7 A, the distance from the spinning needle to the collector is 8 cm, and the spinning rate is 0.7 mL / h. The crystal structure of the obtained PBZ nanofibers at pH = 4.2 is as Figure 4 shown in a, the surface morphology of the precursor nanofibers is as Figure 4 shown in d, and the morphology of the PBZ nanofibers after selecting the final annealing temperature of 750 °C is as Figure 4 shown in i.

[0101] PBZ solution with pH = 5.2:

[0102] Select a 0.8 mol / L PBZ solution:

[0103] Use ammonia water NH3·H2O to adjust the pH value of the solution to 5.2;

[0104] Let the mixed solution stand and age for 24 h for standby;

[0105] Filter the mixed solution with a 0.2 μm filter paper;

[0106] Add it to 0.65 g of PVP and stir at room temperature for 2 h;

[0107] Prepare a nano-precursor of the PBZ solution with pH = 2.2 by electrospinning. The spinning voltage is 11 kV, the spinning current is 1.7 A, the distance from the spinning needle to the collector is 8 cm, and the spinning rate is 0.7 mL / h. The crystal structure of the obtained sample of PBZ nanofibers at pH = 4.2 is as Figure 4 shown in a, and the surface morphology of the precursor nanofibers is as Figure 4 shown in e. Select the morphology of the PBZ nanofibers after the final annealing temperature is 750 °C as Figure 4 shown in j.

[0108] PBZ solution with pH = 6.2:

[0109] Select a 0.8 mol / L PBZ solution:

[0110] Use ammonia water NH3·H2O to adjust the pH value of the solution to 6.2;

[0111] Let the mixed solution stand and age for 24 h for standby;

[0112] Filter the mixed solution with a 0.2 μm filter paper;

[0113] Add it to 0.65 g of PVP and stir at room temperature for 2 h;

[0114] Prepare a nano-precursor of the PBZ solution with pH = 2.2 by electrospinning. The spinning voltage is 11 kV, the spinning current is 1.7 A, the distance from the spinning needle to the collector is 8 cm, and the spinning rate is 0.7 mL / h. The crystal structure of the obtained sample of PBZ nanofibers at pH = 4.2 is as Figure 4 shown in a, and the surface morphology of the precursor nanofibers is as Figure 4 shown in f. Select the morphology of the PBZ nanofibers after the final annealing temperature is 750 °C as Figure 4 shown in k.

[0115] Based on the above process, PBZ precursors with different concentrations and pH values, as well as PBZ nanofibers at different annealing temperatures, can be obtained.

[0116] The preparation process of the PBZ / PVDF flexible pyroelectric thin film used in the examples is exemplarily described as follows:

[0117] PBZ nanofibers with a concentration of 0.8 mol / L, a pH of 4.2, and a final annealing temperature of 750 °C are selected for exemplary illustration;

[0118] PBZ / PVDF flexible pyroelectric thin films with a PBZ nanofiber volume ratio of 0 - 6 vol% are introduced;

[0119] The masses of the PBZ nanofibers weighed are 0 g (0 vol%), 0.029 g (1 vol%), 0.050 g (2 vol%), 0.087 g (3 vol%), 0.115 g (4 vol%), 0.144 g (5 vol%), and 0.173 g (6 vol%) respectively;

[0120] The PBZ nanofibers are dispersed in the DMF solution. The volume of DMF measured for PBZ / PVDF flexible pyroelectric thin films with different volume ratios is constant, all being 8 mL;

[0121] The solution is ultrasonically treated for 1 h to reduce its agglomeration phenomenon;

[0122] The mass of PVDF used for PBZ / PVDF flexible pyroelectric thin films with different volume ratios is constant, all being 0.8 g of PVDF, which is slowly added to the suspension solution;

[0123] The PBZ / PVDF mixed solutions with different volume ratios are magnetically stirred at 30 °C for 20 h to obtain a uniform composite solution;

[0124] The PBZ / PVDF composite solutions with different volume ratios are cast on glass slides, and the film samples are made to have a thickness of 0.3 mm through a highly controllable blade;

[0125] After the PBZ / PVDF composite solutions with different volume ratios are cast, they are placed in a vacuum drying oven at 80 °C for 20 h to evaporate the solvent DMF, and PBZ / PVDF composite films with a thickness of 8 μm are obtained from the glass slides;

[0126] The PBZ / PVDF composite films with different volume ratios are taken out, and gold electrodes are sprayed on their surfaces to obtain PBZ / PVDF flexible pyroelectric thin films for use.

[0127] As the volume doping ratio of the nanofibers increases, from Figure 1 it can be observed that the color of the film gradually changes from nearly transparent to milky white, indicating that the number of PBZ nanofibers contained per unit volume gradually increases, and the color of the fibers themselves changes the transparent state of the entire film. Figure 5Figure a shows the XRD patterns of the composite films at different volume composite ratios. For the composite films with different introduced volume ratios, the characteristic peaks corresponding to PBZ can be measured, indicating that PBZ nanofibers already exist in the PVDF matrix. At the same time, as the volume doping ratio increases, the characteristic peaks of PBZ become higher and higher. Figure 5 Figures b - h show the SEM surface morphologies of the composite films doped with PBZ nanofibers at a volume ratio of 0 - 6 vol%. It can be observed from the figures that as the volume doping ratio increases, the number of PBZ nanofibers contained per unit area gradually increases. The thickness and length of the PBZ nanofibers after ultrasonic treatment are basically uniform, and the dispersion is good. There is no large - area agglomeration phenomenon, and the fibers are coated on the inside and surface of the film in different postures, and the surface quality of the film is good. In order to further observe the quality of the film, its cross - sectional morphology was observed after brittle fracture with liquid nitrogen. Figure 5 Figures i - o show the corresponding cross - sectional morphologies. It can be seen from the figures the brittle - fractured PBZ fibers and the adhered PVDF filaments. At the same time, internal defects caused by solvent evaporation can be seen. A lighter - colored transition layer can be observed at each PVDF nanofiber, that is, the defect left in the transition layer after the solution volatilizes, which makes the nanofibers and PVDF not completely adhered together at the interface. At the same time, there are also some small holes far from the fibers ( Figure 5 Figures p, q). The above - mentioned phenomena are due to the existence of a solvent transition layer between the inorganic material PBZ nanofibers and the organic material PVDF after casting. After the solvent volatilizes, it brings defects in the transition layer. At the same time, far from the fibers, during the heat - preservation process of the film after casting, some small holes merge with each other to form nanoscale holes. The above two factors cause the existence of transition - layer defects and nanoscale hole defects inside the film. At the same time, the introduction of nanofibers has a certain impact on the crystallization of the organic polymer. Figure 5 Figure r shows the infrared test results of the composite film. It can be observed from the figure that the PBZ / PVDF composite film prepared by the casting process mainly exists in the β - phase, with characteristic peaks appearing at 450, 480, 510, 840, 1073, 1280, 1400 cm -1 ; at the same time, there is also a weak non - polar α - phase in the composite film, with characteristic peaks at 763 and 976 cm -1 ; as the PBZ nanofibers are incorporated, the α - peak gradually weakens, indicating that the introduction of nanofibers reduces the α - phase. And as the composite volume ratio increases, the α - peak decreases significantly, indicating that high - volume - ratio nanofibers help to inhibit the α - phase spherulites formed by the disordered arrangement of lamellae; the γ - phase only exists at 1230 cm -1Characteristic peaks show no obvious effect on the γ-phase for composite films with different volume ratios. Compared with the α-phase, the γ-phase has a higher polarity and better stability. Its transformation to the β-phase can only be achieved through stretching or high-pressure annealing. Obviously, the phase transformation cannot be achieved simply by nanofiber composite. Therefore, the γ-phase still exists stably after casting and constant-temperature preservation. At the same time, there is also a part of amorphous state in the film, with characteristic peaks at 600, 880, and 1187 cm -1 For these characteristic peaks, the introduction of nanofibers reduces the amorphous state and helps the crystallization of PVDF. As the volume ratio of the composite nanofibers increases, the characteristic peaks of the amorphous state decrease more significantly, resulting in a higher crystallinity of the matrix and a more obvious promotion of the transformation from the amorphous state to the β-phase, thus improving the ferroelectric properties of the film.

[0128] The P-E loop test of the PBZ / PVDF composite film is as Figure 6 shown in Fig. a. As the volume doping ratio increases (0 vol% → 6 vol%), at the same electric field of 300 MV / m, its ferroelectric properties show an upward trend. This is mainly because the performance of inorganic ferroelectric materials is better than that of organic ferroelectric films. As the filling ratio increases, at the same electric field, the ferroelectric properties of the film with a high volume composite ratio are better than those of the film with a low volume composite ratio. However, this gain effect exists within a certain range. Due to the interfacial incompatibility problem between PBZ nanofibers and the PVDF matrix, as the volume composite ratio increases, more interfacial defects will be left after the volatile organic solvents. The defects caused by the poor interfacial compatibility become more and more obvious, which can be observed from Figure 5 Figs. i-q. The increase in internal defects will affect its breakdown field strength, causing the breakdown field strength to decrease as the volume composite ratio increases. The realization of a high pyroelectric effect depends on a high breakdown electric field. Therefore, an appropriate volume doping ratio can not only obtain a relatively high breakdown field strength but also a relatively large polarization value change rate, thus obtaining a relatively high pyroelectric effect. Figure 6 Fig. b shows the ferroelectric test of the composite film under the maximum electric field. From the data, when the volume composite ratio is 3 vol%, the composite film has the best ferroelectric properties. The maximum polarization value and the remanent polarization value are 9.69 and 5.31 μC / cm2 respectively, and at the same time, its breakdown field strength is the largest, which is 401 MV / m. Figure 6 Fig. c shows the variation of the maximum polarization value, remanent polarization value, and breakdown electric field of the composite film with the volume composite ratio. When the volume composite ratio is 6 vol%, its breakdown field strength is only 296 MV / m, lower than the breakdown field strength of the pure PVDF film, which is 378 MV / m. However, its maximum polarization value is 4.80 μC / cm2, higher than the polarization value of the pure PVDF film, which is 4.36 μC / cm2, indicating that the promoting effect of nanofibers still exists. Figure 6d is the Weibull distribution of the composite film (in order to distinguish the fitting curves of the corresponding values, the abscissa is incremented by 3 in turn). The magnitude of the inverse pyroelectric performance of the film is related to the breakdown field strength. A larger breakdown field strength is a prerequisite for obtaining a high inverse pyroelectric effect, and it will also improve the reliability of the material device. However, the dielectric breakdown field strength of the composite film has the characteristics of strong randomness, large difference, and uneven distribution. Its breakdown field strength can be statistically distributed to study its law. The slope of the straight line is the shape parameter β, and its value is related to the distribution of the dielectric breakdown field strength. When β > 1, it indicates that the experimental data is applicable to the Weibull distribution. From the figure, for the composite films with different volume ratios, their β values are all greater than 1, indicating that the experimental data conforms to the Weibull distribution. At the same time, the corresponding size parameter α can also be obtained, which characterizes its corresponding average expectation, that is, the average breakdown electric field. The β values of the 3vol% and 6vol% composite films are 5.16 and 2.36 respectively. The statistical results show that the reliability of the 3vol% volume ratio is higher. Through lnα, their average breakdown field strengths are 428 MV / m and 237 MV / m respectively. Therefore, the composite film with a 3vol% volume ratio has a higher breakdown field strength and polarization state. This shows that at a low volume composite ratio, the nanofibers help to moderately increase its breakdown field strength and enhance its ferroelectric properties. At this time, the defects caused by the volatilization of the organic solvent do not play a role in reducing the performance, and the gain effect of the nanofibers dominates. When the volume ratio continues to increase when the performance of the composite film reaches the peak, the defects caused by the volatilization of the organic solvent dominate, and the gain effect of the nanofibers is inhibited. Therefore, its breakdown field strength decreases and its ferroelectric properties decline.

[0129] In order to explore its dielectric dispersion state, Figure 7 Figure 5 shows the variation relationship between the dielectric constant and dielectric loss of the 3vol% PBZ / PVDF composite film at 173 - 373K and different frequencies (100 - 1000 kHz). The peak with the maximum value in the dielectric loss is the depolarization temperature T1, corresponding to the phase transition temperature of the PBZ ceramic from the antiferroelectric phase to the ferroelectric phase. This phase transition causes a sharp change in the dielectric loss and is manifested in the composite film. At 285K, its dielectric constant is 6.13 and the dielectric loss is 0.21 (1000 kHz). By substituting Ba atoms for Pb atoms, the phase transition temperature from the antiferroelectric phase to the ferroelectric phase reaches near room temperature, so there is a phase transition temperature at room temperature. A large inverse pyroelectric effect will be generated in the temperature range where there is a phase transition. Therefore, studying the inverse pyroelectric effect near this temperature has certain significance and value.

[0130] When a DC electric field is instantaneously applied to the composite film, the dipoles flip and align towards the direction of the electric field, the degree of order of the material increases, the entropy value of the composite film decreases, and thus the temperature rises rapidly; when the electric field applied to the two electrodes is instantaneously removed, the directional flipping of the dipoles weakens, so the degree of order decreases, the entropy value of the nanocomposite film increases, and at this time the material temperature decreases. The composite film exhibits typical pyroelectric and reverse pyroelectric effects (the red arrow represents the decreased heat flux (Φ) caused by the heat reduction when the DC electric field is removed. For example, when the temperature is -30 °C, Figure 8 a shows the heat flux (Φ) changes of the 3 vol% PBZ / PVDF composite film under different electric fields, Figure 8 b shows the locally magnified heat flux (Φ) changes, with the electric field being 150 MV / m. The isothermal unit heat (Q) is calculated by TA universal analysis software, and its equation is where C p is the specific heat capacity of the test sample, T is the ambient temperature, and the adiabatic temperature change (ΔT) and the isothermal entropy change (ΔS) can be calculated. When the ambient temperature is the same, as the electric field increases, the heat (Q) caused by the removal of the electric field gradually increases, and the reverse pyroelectric effect becomes more obvious. For example, when the ambient temperature is -30 °C, Figure 9 a and b show that the heat Q and ΔT gradually increase with the increase of the applied electric field. At the same time, the temperature application window of the reverse pyroelectric material is also a very important parameter. When the external electric field is applied or removed, the corresponding heat flux (Φ) signals at different temperatures also change, and the relevant EC parameters are calculated as Figure 9As shown in c and d. When the external electric field is constant, the inverse pyroelectric performance of the 3vol% PBZ / PVDF composite film changes with the surrounding temperature. This is mainly because the flipping of dipoles is affected by temperature, resulting in changes in △T and △S. At an electric field of 150 MV / m and -30 °C, △T and △S of the composite film are 13.99 K and 52.70 J / kg·K respectively, while the corresponding parameters of the pure PVDF film are 4.29 K and 20.65 J / kg·K; at 10 °C, △T and △S of the composite film reach the minimum values of 3.35 K and 10.84 J / kg·K, and △T and △S of the pure PVDF film are 1.34 K and 5.55 J / kg·K respectively; when the temperature rises to 70 °C, the corresponding performance also increases accordingly. At this time, △T and △S of the composite film are 5.08 K and 13.56 J / kg·K, and the corresponding △T and △S of the pure PVDF film are 2.45 K and 8.36 J / kg·K. The introduction of nanofibers can significantly improve the inverse pyroelectric performance of its matrix material. Although the applicable window of the composite film is narrow, when the external temperature is constant, the PBZ / PVDF composite film exhibits high inverse pyroelectric performance, and its price is lower than that of PVDF copolymers (P(VDF-TrFE) and P(VDF-TrFE-CFE)). Under certain conditions, it is an excellent choice and is beneficial to the practical application of inverse pyroelectric materials.

[0131] The above are only the preferred embodiments of the present invention and are not used to limit the present invention. Those skilled in the art can make various modifications or equivalent replacements to the present invention within the essence and protection scope of the present invention. Such modifications or equivalent replacements should also be regarded as falling within the protection scope of the technical solution of the present invention.

Claims

1. A flexible inverse pyroelectric film, characterized in that: The invention comprises an inorganic nanofiller and an organic polymer, wherein the inorganic nanofiller is PBZ lead barium zirconate nanofiber, and the organic polymer is a PVDF polyvinylidene fluoride flexible substrate, wherein the chemical formula of the PBZ lead barium zirconate nanofiber is: Pb 1-x Ba x ZrO3, wherein X is 0.

2.

2. A process for preparing a flexible inverse pyroelectric film, characterized in that: The steps include: Take lead acetate trihydrate Pb(CH3COO)2·3H2O and barium acetate C4H6BaO4 and dissolve them in glacial acetic acid CH3COOH and deionized water according to the corresponding molar ratio of the elements, stir at a certain temperature and then cool to room temperature, add a certain amount of lead acetate trihydrate Pb(CH3COO)2·3H2O twice to obtain a first solution; Take the corresponding mass of zirconium n-propoxide solution C 12 H 28 O4Zr is dissolved in 2-ethylene glycol methyl ether C3H8O2, and after stirring at room temperature, acetylacetone C5H8O2 is added and stirred to obtain a second solution; The first solution and the second solution are mixed and stirred, and then the final PBZ lead barium zirconate solution concentration and pH value are adjusted by deionized water and acid-base solution, and after standing and aging, filtered through 0.2 μm filter paper, and then polyvinyl pyrrolidone is added, stirred, and electrospinning is performed to obtain a PBZ nano precursor; The PBZ nano precursor is placed in a sintering furnace, an annealing process is performed to obtain the final product, and the PBZ nanofibers are obtained by cooling to room temperature; PBZ nanofibers were dispersed in N, N-dimethoxyformamide, PVDF powder was slowly added after ultrasonic treatment, and a composite solution was obtained after magnetic stirring; The composite solution is cast on a glass slide to form a thin film sample. The film sample is left with a corresponding thickness by a highly controllable scraper. After the casting is completed, it is placed in a vacuum drying oven at 80°C for 20 hours to evaporate the solvent DMF. The composite film is obtained from the glass slide, and gold electrodes are sprayed on the surface of the composite film to obtain a flexible inverse pyroelectric film.

3. The process for preparing a flexible inverse pyroelectric film according to claim 2, characterized in that: The stirring temperature of the lead acetate trihydrate Pb(CH3COO)2·3H2O and the barium acetate C4H6BaO4 is 50-70°C, and the stirring time is 20-40 minutes.

4. The process for preparing a flexible inverse pyroelectric film according to claim 2, characterized in that: The amount of lead acetate trihydrate Pb(CH3COO)2·3H2O added is 15-25% of the first mass.

5. The process for preparing a flexible inverse pyroelectric film according to claim 2, characterized in that: The zirconium n-propoxide solution C 12 H 28 The stirring time of O4Zr dissolved in 2-ethylene glycol methyl ether C3H8O2 is 5 to 15 minutes, and the stirring time of adding acetylacetone C5H8O2 is 20 to 40 minutes.

6. The process for preparing a flexible inverse pyroelectric film according to claim 2, characterized in that: The stirring time of the first solution and the second solution is 1.5 to 2.5 hours, the concentration of the PBZ lead barium zirconate solution is 0.2 to 0.8 mol / L, and the pH value is 2.2 to 6.

2.

7. The process for preparing a flexible inverse pyroelectric film according to claim 2, characterized in that: The electrospinning parameters are as follows: spinning voltage is 11 kV, spinning current is 1.7 A, the distance from the spinning needle to the collector is 8 cm, and the spinning rate is 0.7 mL / h.

8. The process for preparing a flexible inverse pyroelectric film according to claim 2, characterized in that: The annealing process is specifically as follows: first, the temperature is increased from room temperature to 350°C at a heating rate of 3°C / min, and kept at 350°C for 30 minutes, then the heating temperature is increased from 350°C to 450°C at a heating rate of 5°C / min, and kept at 450°C for 60 minutes, and finally the temperature is increased from 450°C to the final temperature at a heating rate of 5°C / min and kept at a constant temperature for 3 hours, and the final annealing temperature is from 550°C to 750°C, the temperature difference is 50°C, and the holding time is 3 hours.

9. The process for preparing a flexible inverse pyroelectric film according to claim 2, characterized in that: The amount of PBZ nanofiber added is 1-6 vol% of the total volume, the ultrasonic treatment time is 0.5-1.5 h, the magnetic stirring temperature is 30° C., and the stirring time is 20 h.

10. The process for preparing a flexible inverse pyroelectric film according to claim 2, characterized in that: The film sample has a thickness of 0.1-0.4 mm, a vacuum drying oven temperature of 80° C., and a drying time of 20 h.

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